Does anyone know where the location of this key is or what side quest is apart of it perhaps? I unlocked the master lock on the outside and mined the Stalhrim to the next door, but that one requires a key. uggggg Bethesda why can't I just explore without the necessity to do side quests?
The key is in one of the Deathbrand armor chests. You can get those by starting the Deathbrand quest. The armor is worth doing the quest! Travel up to the North East side of the island to a camp called Haknir's Shoal and you can start the quest from there.